1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission that quickly pressurizes and quickly vents the cylinder of a clutch or servo through operation of a solenoid-operated control valve. More particularly, the invention pertains to a control system which pressurizes a brake servo at different pressure levels depending on the gear ratio in which the transmission operates.
2. Description of the Prior Art
An automatic transmission for automotive use includes planetary gearsets whose elements are hydraulically connected or held in order to produce selected ratios of the engine crankshaft speed to the speed of the output shaft of the transmission. Automatic gear change transmissions may be controlled by electronic means which produce analog current signals that operate solenoid valves. These valves connect hydraulic line pressure to selected clutches and brakes, or vent the hydraulic cylinder of these elements to atmosphere. The control system should operate such that, in the event of a control system malfunction, the transmission may operate in at least one forward speed ratio and in reverse drive in order to allow the vehicle to be driven to a service station for repair.
Fluid capacity modulator valves and accumulators operate to smooth gear ratio upshifts and downshifts by controlling the rate at which the associated clutch or servos are applied. When an upshift begins, the capacity modulator valve produces a reduced or regulated pressure which is applied to the operative clutch and to the spring of an accumulator. The accumulator is filled and pressure regulation continues until it is fully pressurized. Thereafter, regulation ceases and line pressure is directly applied to the clutch cylinder to complete the engagement. Such hydraulic systems provide gradual or rapid pressure build-up depending upon the conditions and the requirements indicated by movement of a control valve.